This application relates generally to printed circuit board construction for supporting carrier mounted integrated circuit chips. More particularly, this application is directed to a particular circuit board construction for use in connection with leadless ceramic chip carriers intended for operation under severe environmental conditions and in which the thermal coefficient of expansion of the ceramic carrier is matched to that of the supporting board.
It is common practice to mount integrated circuit chips on various types of chip carriers to meet a variety of special requirements. These requirements include ease of mounting the carriers on circuit boards to form a more complex board assembly in a manner which is both convenient and inexpensive. The circuit board must also cooperate to adequately remove heat from the chip under severe thermal conditions which may be encountered in operation. Physical contacts between the various components of the assembly must permit expansion and contraction with temperature changes without causing excessive stresses at critical points in the assembly. The composite assemblies must, in addition, be light in weight, compact in size and be easily fabricated using known techniques.
Prior art printed circuit board assemblies typically comprise some combination of metals, plastic and ceramics, each of which compromises one aspect of functionality to emphasize another. For example, high electrical and thermal conductivity metals such as copper and aluminum, while essentially providing excellent heat removal characteristics, differ substantially from ceramics in thermal expansion. These metals thus cause thermal fatigue, undue stresses and premature failure at the intermediate interfaces such as at solder joints, etc. Other metals may be selected which better match the thermal characteristics of the ceramic but do not provide the same high degree of thermal sink properties, electrical conductivity, or are less easily fabricated to form the assembly. While plastics may provide easy fabrication into various configurations, they do not provide for sufficient heat removal, electrical connections or hermeticity. In short, while various combinations and types of materials such as plastics, metals and ceramics have been used to fabricate circuit board assemblies with specified functional characteristics, each of these materials has its drawbacks; no known combination is a panacea.
Where the circuit assembly is to meet the requirements of a demanding environment, ceramic chip carriers have been favored in use. In particular, such ceramic carrier arrangments are typically used or specified for military applications and for use in industrial computer and telecommunication applications, particularly where hermeticity is desired. Additionally, where high density surface mounting is mandatory chip carriers of the hermetically sealed leadless type are most frequently used. Such packages require thermally matched or controlled circuit mounting boards, as alluded to above. Specifically, either the temperature environment must be controlled within narrow specified limits or the components must be selected to avoid adverse effects attendant to stresses caused by large fluctuations in working condition temperatures.
One particular stress which must be addressed in such arrangements is the large thermal mismatch between the chip carrier's alumina body (with a thermal coefficient of expansion of approximately 6 ppm/.degree.C.); a supporting copper thermal heat sink (with an approximate thermal coefficient of expansion of 17 ppm/.degree.C.); and, for example, intermediately located laminates of various kinds with typical thermal coefficients of expansion in the approximate range of 3 to 14 ppm/.degree.C.). These differences in the coefficients of thermal expansion of the ceramic carrier, underlying laminate and supporting heat sink core result in stresses at the electrical connections between the chip carrier leads or terminals on the body of the ceramic carrier and the conductive runs on the laminate which supports the package and elsewhere in the assembly.
There are three general solutions to the above discussed problem. The first is a board material that will nearly match the thermal coefficient of expansion of the alumina carrier. The second is to put leads on the ceramic carrier, but this adds expense to an already costly unit. A third is to replace the leadless ceramic carrier with a plastic leaded unit that contains some sort of humidity protection.
The military has concentrated its efforts on the first method, i.e., a construction which approximately matches the ceramic's thermal coefficient of expansion to that of the underlying structure. A popular method to accomplish this is to use sandwiched layers of either copper-Kevlar-copper, copper-molybdenum-copper, or epoxy-graphite as the inner core of the multilayer structure. This diminishes the problem of thermal expansion mismatch since the inner core's thermal characteristics dominate the composite thermal coefficient of expansion and match that of the carrier's ceramic. Another variation is to use rigid composites, such as polyimide reinforced Kevlar or quartz, to achieve the low thermal coefficient of expansion required. These materials have sufficiently low thermal coefficients of expansion to keep shear strains of the solder joint to a minimum. However, routing and drilling of quartz in Kevlar fabric-reinforced composites are extremely difficult. In addition, microcracking of the brittle polyimide matrix resulting from excessive radial expansion of Kevlar fibers has stalled the widespread acceptance of polyimide Kevlar. Accordingly, it would be desirable to provide a circuit board assembly which, in addition to meeting the previously mentioned function goals, also better matches the thermal expansion coefficient of a ceramic chip carrier to that of the underlying inner core support structure while concurrently maintaining or improving heat sink performance.
This application is, therefore, directed to an improvement in the above-noted technique of matching the thermal coefficient of expansion of the ceramic chip carrier body to that of the inner core of a multilayer structure. By varying the structure of the inner core, a better match with the thermal coefficient of expansion of the ceramic chip carrier is achieved, while concurrently providing a more efficient thermal heat sink.